It depends what level of fact checking is needed. Watson is well-suited for answering questions like "What year was Obama born?", because the answer is unambiguous and also fairly likely to be found in a database. I would be very surprised if Watson could fact check a statement like "Putin has absolutely no respect for President Obama", because the context needed to evaluate such a statement is not so easy to search for and interpret.

Yes, because Watson's corpus doesn't contain people lying. On the other hand, for political fact-checking the corpus is going to have tons of lies, half-truth, and BS.

I think the standard for accuracy would be very different. If Watson gets something right you think "Wow that was so clever", if it's wrong you're fairly forgiving. On that other hand, I feel like if an automated fact checker got even 1/10 things wrong it would be subject to insatiable rage for doing so. I think specifically correcting others is the situation in which people would have the highest standard for accuracy.

And that's before you get into the levels of subjectivity and technicality in the subject matter which something like Watson would never be subjected to.